The U.S. Department of Transportation shut down the discount bus service Sky Express after four people were killed and 50 others injured when a bus bound for New York City crashed in Virginia.

Watch "World News with Diane Sawyer" for more on this story tonight on ABC.

Regulators issued a statement saying that the bus service violated several federal safety regulations.

"This company should not have been operating," United States Secretary of Transportation Ray LaHood told ABC News. "We take responsibility for bus safety and we need to do better."

The bus was heading northbound on Interstate 95 early Tuesday when it hit an embankment and overturned 30 miles north of Richmond, Va. The driver, Kin Yiu Cheung, was charged with reckless driving after police found that fatigue could have played a role in the accident. He was later released on bond.

Sky Express was last inspected on April 7 amid concerns within the Department of Transportation. Shortly after that inspection, the department issued a "proposed unsatisfactory rating."

Sky Express buses have been involved in four crashes with an injury or fatality during a two-year period that ended on May 20, according to Federal Motor Carrier Safety Administration records. The company's drivers have been cited for 17 unsafe-driving violations since 2009, including 11 for speeding.

In the last two years, Sky Express was also considered 62.9 percent worse than the bus and truck companies that had a similar number of inspections in the unsafe driving category, and was 86.2 percent worse when it came to driver fatigue.

The company was also a staggering 99.7 percent worse than companies who had a similar number of inspections when it came to driver fitness. It had 24 violations in that category, 14 of which were having non-English speaking drivers. In one case, a driver did not have a commercial driver's license.

Seven hundred fifty million passengers board 3,200 buses every year in the U.S. and Canada, according to the American Bus Association.

"There are thousands of bus operators out there, but that's not an excuse," Robert Sumwalt, an NTSB board member, told ABC News. "The traveling public deserves, each and every time they get on a bus, they're entitled to and they deserve a level of safety. And currently that's not always happening in all cases."

Sky Express is part of a booming sub-industry of inexpensive buses on the East Coast that offer cheap fares, free wireless Internet and convenient routes.

But a string of fatal accidents have prompted calls for tighter regulation.

"How many deaths do we have to have before the bus companies are going to start saying, 'Maybe we don't need more time. Maybe we should start doing something about this,' with out without the government telling them to," said Sen. Sherrod Brown, D-Ohio. "Sometimes they need government mandates."

Fifteen people were killed in March when another company's bus crashed returning to Chinatown from a Connecticut casino. Last month, a tour bus carrying fans home from a soccer game crashed into a pickup truck in Washington, triggering a three-vehicle crash that killed two people and injured 15 others.

Peter Pantuso, the president and chief executive officer of the American Bus Association, said that there should be bigger consequences for these companies.

"We're glad to see that they were put out of service," he said. "What we would really like to see is, if this company has the safety record we're looking at, companies like this should be completely shut down."

Under law, companies have 46 days to appeal the decision to put a carrier out of service and show why they should not be shut down. That can be extended by 10 days if the Dept. of Transportation needs more time to go through the evidence to decide whether to order them to close up for good.

Sky Express was given an unsatisfactory rating on April 12 and was given the 45-day period to appeal the Department of Transportation's decision. The company submitted an appeal on May 11, and was initially denied.

However, the Transportation Department then found drug, alcohol and hours-of-service concerns with the company, and extended its review period 10 days past the May 28 deadline for an official decision.

Because of the crash, LaHood decided to end the appeals process altogether and immediately put Sky Express out of service.

"There is no excuse for delay when a bus operator should be put out of service for safety's sake," he said in a statement. "On my watch, there will never be another extension granted to a carrier we believe is unsafe."

Pantuso said that there should be more inspections done on a regular basis.

"They had this long history of violations through 2009," he said of Sky Express. "Why weren't they put out of service?"